You might try unplugging the speakers from power for a couple of minutes and rebooting your router. You can also try changing the wireless channel on your router. If possible, it might be beneficial for you to connect one Sonos device directly to your router to create SonosNet which can give you more reliable network performance from all of your Sonos speakers.

Are both Pairs grouped into one single Group? Two individual Groups won’t stay in sync.

If they are in a single Group and you have tried the above suggestions you should wait for the out-of-sync to happen again and submit a diagnostic, keep the number and contact Sonos Support with it for advanced internal troubleshooting that we users can’t do.

Thank you very much for replying. Why won't they stay in sync with two individual Groups? I have done that multiple times earlier before I moved to at new place. That’s one of the points of having Sonos that you can hear the same music all over the house - or choose individual sources…. Or am I wrong?

Why not have all speakers as one group?

You can have the same music all over the house but if you want the same music at the same time then you have to put all the speakers into a single group.

If you have two groups playing the same stream they are not synchronized and will usually drift over time. How much drift is hard to predict, partly based on network conditions but also based on how the stream is generated.